Novel extracellular role of REIC/Dkk-3 protein in PD-L1 regulation in cancer cells

Abstract The adenovirus-REIC/Dkk-3 expression vector (Ad-REIC) is a clinical study's focus, with the goal of using this gene to exploit the quenching of several cancer types. REIC/DKK- 3 gene's cancer-suppressing mechanisms depend on multiple pathways that exert both direct and indirect effects on cancers. The direct effect is triggered by REIC/Dkk-3-mediated ER-stress that causes cancer-selective apoptosis, and the indirect effect can be classified in two ways: ( i ) Ad-REIC-mis-infected cancer-associated fibroblasts that lead to the production of IL-7, an important activator of T cells and NK cells, and ( ii ) the secretory REIC/Dkk-3 protein that promotes dendritic cell polarization from monocytes. With these unique features, Ad-REIC exerts an effective and selective prevention of cancer as if it were acting as an anticancer vaccine. However, the question of how REIC/Dkk-3 protein leverages an anticancer immunity has remained to be answered. We herein report a novel function of the extracellular REIC/Dkk-3 that regulates an immune checkpoint via a modulation of PD-L1 on the cancer-cell surface. We identified novel interactions of REIC/Dkk-3 with the membrane proteins C5aR, CXCR2, CXCR6 and CMTM6. These proteins all functioned to stabilize PD-L1 on the cell surface. Due to the dominant expression of CMTM6 among the proteins in cancer cells, we focused on CMTM6 and observed that REIC/Dkk-3 competitively bound with CMTM6 to PD-L1, which induced the liberation of PD-L1 from CMTM6 binding; the released PD-L1 immediately underwent endocytosis-mediated degradation. These results increase our understanding of not only the physiological nature of the extracellular REIC/Dkk-3 protein but also the Ad-REIC-mediated anticancer effects.